1. Field of the Invention
The invention relates generally to audio amplification systems, and more particularly to systems and methods for minimizing performance degradation due to component mismatch in the feedback path of a digital PWM amplifier feedback loop.
2. Related Art
Practical audio power amplifiers using Pulse Width Modulation (PWM) have been known since the mid 1960s. In amplifiers from that era, a pulse train was generated by comparing a voltage representing the incoming audio signal with a reference waveform, typically a triangular wave or sawtooth wave, with a frequency in the range 50 kHz-200 kHz. The comparison yielded a 2-level rectangular wave having the same frequency as the reference waveform, and with a mark:space ratio varying in sympathy with the audio. The rectangular wave was amplified to the desired power level and then passively lowpass filtered to remove most of the high-frequency components of the rectangular wave, leaving its average level, which follows the audio, to drive a load such as a loudspeaker.
It is possible to obtain extremely good performance when such amplifiers are run ‘open-loop’, that is without feedback, but it is an expensive solution, since the amplifier's performance is critically dependent on the quality of the output stages and the power supply. To alleviate these dependencies, the trend in the 1970's, and subsequently, has been to incorporate feedback. One simple way to incorporate feedback in an amplifier that compares the audio with a triangle wave is to replace a fixed triangle wave by a sawtooth wave that is obtained by integrating the substantially rectangular waveform. Analysis shows that this is an effective means of providing feedback. Moreover since the feedback is tightly integrated into the pulse width modulation itself, stability problems typically associated with feedback do not arise.
Amplifiers as described above have sometimes been called ‘digital’ in the popular press, but we shall describe them as ‘analog’, because the timings of the edges of the rectangular waves can vary continuously in sympathy with the audio. We shall reserve the word ‘digital’ for an amplifier in which the edge timings are quantized, so that the edge timings can be represented digitally and the edges can be generated by counting pulses produced by a high-precision, high-frequency clock, such as a crystal oscillator. This principle was proposed by Sandier (Sandier, M., “Towards a Digital Power Amplifier” Audio Eng. Soc Preprint Number: 2135, September 1984,) who also realized that the apparent need for a clock frequency in the gigahertz region could be avoided by the use of oversampling and noise shaping. Several commercial products are now available that use this principle (see, for example, Harris, S., Andersen, J., and Chieng, D., “Intelligent Class D Amplifier Controller Integrated Circuit as an Ingredient Technology for Multi-Channel Amplifier Modules of Greater than 50 Watts/Channel” Presented at the AES115th Convention 2003 October 10-13 New York, Audio Eng. Soc. preprint # 5947.)
The digital principle brings precision to the generation of the PWM waveform, but the power amplification, typically accomplished by MOSFET (Metal Oxide Silicon Field Effect Transistors) power switches, remains a fundamentally analog process, and as such is vulnerable to non-ideal component behavior. There is a distortion associated with the switching called “dead-time distortion”, and there is dependency on the power supply, just as with the original analog PWM amplifiers. Without feedback or other compensation, the gain of the output stage will be directly proportional to the supply voltage. This precludes the use of an inexpensive non-regulated power supply in low-cost applications, or condemns the system to relatively poor performance.
Andersen et al., in U.S. patent application Ser. No. 11/324,132 (“Andersen et al.”), illustrated how to construct a digital PWM amplifier utilizing feedback techniques to improve performance. The described system uses analog-to-digital converters (ADCs) to sample and digitize the amplifier's analog output and/or the power supply for use in the feedback signal processing. This amplifier system utilized a PWM controller and independent ADCs for sampling the analog output and power supply.